Fixed USB-CDC buffering. Now serial output performs much better.

hardware/arduino/sam/cores/arduino/USB/CDC.cpp

@@ -228,7 +228,6 @@ size_t Serial_::write(uint8_t c)
 
 		if (r > 0)
 		{
-			USBD_Flush(CDC_TX);
 			return r;
 		} else
 		{

hardware/arduino/sam/cores/arduino/USB/USBCore.cpp

@@ -397,6 +397,12 @@ static void USB_ISR(void)
 
 		udd_ack_fifocon(CDC_RX);
 	}
+
+	if (Is_udd_sof())
+	{
+		udd_ack_sof();
+		USBD_Flush(CDC_TX);
+	}
 #endif
 
 	// EP 0 Interrupt

hardware/arduino/sam/system/libsam/source/uotghs_device.c

@@ -115,7 +115,7 @@ void UDD_Attach(void)
 
 	// Enable USB line events
 	udd_enable_reset_interrupt();
-	//udd_enable_sof_interrupt();
+	udd_enable_sof_interrupt();
 
 	cpu_irq_restore(flags);
 }